OpenGL allows mixing and matching depth and stencil renderbuffers in
framebuffer objects while the hardware really only supports interleaved
depth/stencil buffers.  This makes for some tricky buffer management.

An extra wrinkle is the situation where the user allocates a 16bpp depth
texture or renderbuffer then tries to render to it along with a stencil
buffer.  We'd have to promote the 16bpp Z values to 24-bit Z values and
mix in the stencil values to setup the depth/stencil renderbuffer.

There's no support for that now, so always allocate 32bpp depth textures/
renderbuffers for now.